In FM broadcasting systems, the Left (L) and Right (R) channels of a stereo audio signal are transmitted as an L+R signal, which carries the “mono” information, and an L−R signal, which carries the “stereo” information. FIG. 1 depicts a typical spectrum of a composite baseband signal used in an FM broadcasting systems. This composite baseband signal is also sometimes referred to as the FM stereo multiplex (MPX) signal. As reflected in FIG. 1, the L+R signal is transmitted as baseband audio in the range of 30 Hz to 15 kHz while the L−R signal is modulated onto a 38 kHz double-sideband suppressed carrier (DSBSC) signal occupying the baseband range of 23 to 53 kHz. As also reflected in FIG. 1, a 19 kHz pilot tone, at half the 38 kHz sub-carrier frequency and with a precise phase relationship to it is also generated. This is transmitted at 8-10% of the overall modulation level and used by a stereo FM receiver to regenerate the 38 kHz sub-carrier with the correct phase. In addition, a 57 kHz sub-carrier (phase locked to the third harmonic of the stereo pilot tone) may be used to carry a low-bandwidth digital Radio Broadcast Data System (RBDS) signal. The composite baseband signal may further include a DirectBand signal as shown in FIG. 1, as well as other signals that are not shown in FIG. 1. The composite baseband signal is used to modulate the FM transmitter.
A stereo FM receiver will add the L+R signal to the L−R signal to recover the L signal and will subtract the L−R signal from the L+R signal to recover the R signal. It has been observed that the L−R signal has a signal-to-noise ratio (SNR) that is about 20 dB worse than the L+R signal. Thus, as the carrier-to-noise ratio (CNR) decreases, the higher noise level in the L−R signal dominates the perceived noise in the reconstructed L and R signals. To address this issue, in conventional FM receiver designs, the proportion of the L−R contribution used in reconstructing the L and R signals (or the “weight” placed on the L−R signal when calculating a weighted sum of the L+R and L−R signals) is gradually reduced with decreasing CNR so as to reduce the noise level in the reconstructed L and R signals. This is called “stereo blending,” and it achieves a reduced noise level at the price of reduced stereo separation. As the CNR continues to decrease, eventually the FM receiver output effectively collapses to only the L+R, or mono, signal. When the CNR crosses below 12 dB or so, an impulsive type of noise appears in the L+R signal and is perceived as “static.” Hence, as the CNR decreases to a relatively low level, the reduced stereo separation, the increased hissing background noise, and the appearance of noise pulses (static) all degrade the perceived audio quality of the FM receiver output signal significantly. Any attempt to address these problems should not involve changing the FM transmitters due to the large installed base of FM receivers in use today.
What is needed, then, is a system or method that can operate in an FM receiver and that can improve the audio quality at low CNRs by enhancing the stereo separation and reducing the hissing background noise and static in the FM receiver output audio signal.
The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.